1. Field of the Invention
This invention relates generally to the deployment and configuration of tubular connections between the bottom of a body of water and a vessel floating on the surface to permit conveyance of liquids or gases under pressure while the vessel is maintained nearly stationary or with only limited movement.
2. Background Art
In offshore oil and gas fields, so-called risers are employed to convey fluids between the seabed and a vessel on the surface of the sea. These risers consist of a conduit or combinations of conduits arranged so that the conduits can deflect sufficiently to remain securely connected even though the vessel is displaced in horizontal and vertical directions due to the combined actions of wind, waves, and currents on the vessel. The vessel may be moored to the seabed through anchor and chain connection, or it may be kept on station by means of a dynamic positioning system of thrusters on the vessel operated to continually counteract the wind, wave, and current forces.
FIGS. 1-4 illustrate typical riser assemblies according to the known art, with the same elements in each figure being designated by the same reference numeral. In FIG. 1, a pipeline 10 on the sea bed 11 connects through a pipeline end manifold 12 to a buoyant rigid pipe riser 13, which can pivot to a limited degree about the manifold 12. The riser 13 connects to a vessel 14, such as a semi-submersible platform, through a flexible pipe jumper 15 (for example, of the type manufactured by Coflexip) to complete the fluid path between the seabed pipeline 10 and the vessel 14. The jumper 15 hangs in a catenary shape between the upper end of the riser 13 and the vessel 14. The catenary of the jumper 15 and the pivoting motion of the riser 13 combine to permit substantial displacement of the vessel 14 in both the vertical and horizontal directions yet still maintain a secure fluid path. The illustrated semi-submersible platform vessel 14 is also moored to the seabed by anchor chains 16 and piles 17.
FIG. 2 shows another example of a conventional riser arrangement in which a flexible pipe 18, having a portion 19 that rests on the seabed 11 and a catenary portion 20, provides a direct connection between the pipeline end manifold 12 and a vessel 21, such as a tanker or a special purpose vessel known as a floating storage and off-loading (FSO) or floating production, storage and off-loading (FPSO) vessel. In this example, the vessel 21 is shown as free floating, being maintained on station by thrusters 22 without separate anchor chains.
FIGS. 3 and 4 show still another known technology whereby a flexible riser pipe 23 connects the pipeline end manifold 12 to a floating vessel 24 through a structural swivel turret 25 rotatably mounted in the bottom of the vessel. In FIG. 3, a plurality of buoyancy tanks 26 spaced along a section of the riser 23 support the riser in an S-curve to provide additional flexibility. In FIG. 4, the plurality of buoyancy tanks 23 are replaced by a single larger buoyancy tank 27 moored by a tether 28 to a clump weight 29 on the seafloor. The tethered buoyancy tank 27 also forces the riser to assume an S-shape in the water, and it has the advantage over the arrangement of FIG. 3 of providing a more positive control of the shape of the riser 23 when fluids of different specific gravities being transferred through the riser change the buoyancy of the pipe. As in the example of FIG. 2, the vessel 24 may be maintained on station by thrusters (not shown), or the vessel may be moored by anchors and chains as in FIG. 1.
The prior art riser technologies illustrated in FIGS. 1-4, all rely on flexible pipe, which may be unsuitable for certain oil field operations, such as pumping down tools. In addition, the existing technologies rely on the strength of the pipe itself to resist axial forces imposed on the riser. Changes in the specific gravity of the contents of the riser or the negative buoyancy of the pipe itself may overstress the pipe axially when the water is very deep, say 1,000 meters or more. The existing technology also does not permit very large motions of the surface vessel when the water is shallow (i.e., only slightly deeper than the draft of the vessel) without danger of damaging the riser either by bending it more sharply than the damage bending radius of the pipe or by chafing it against the vessel, the seabed, or both.